The development of materials that are sufficiently strong and large enough for structural applications is an important and challenging problem. Traditionally, metals are preferred for these applications because of their combined strength and ductility. Metals can be made stronger using various methods that refine the grain size of the material from a coarse grain size to an ultrafine grain (UFG) size of a few microns or less.
Although most high-strain deformation processing techniques, such as extrusion, rolling, and drawing, provide materials with refined grain sizes and improved strength, they do not preserve the dimensions of the original workpiece. One or more dimensions of the workpiece are continuously reduced, which not only limits the obtainable strain, but also eventually transforms the workpiece to a product having a final geometry of a plate, foil, or wire, which limits its structural applications.
A recently developed technique known as Equal Channel Angular Extrusion (ECAE) has been used to provide an ultrafine-grained metal, alloy, plastic, or ceramic product from a coarser grained workpiece without significantly changing the dimensions of the workpiece. Briefly, the ECAE method involves pressing a metal workpiece through a die having two channels that are equal in cross-section and that intersect at an angle .PHI.. During the pressing, the workpiece undergoes severe shear deformation that refines the grain size and improves strength. Thus, the advantage of the ECAE method is the combination of improving the strength of a workpiece by grain refinement while maintaining its dimensions. The ECAE method may also be combined with cold working procedures such as cold rolling to produce refined, elongated grains.
Current limitations of ECAE hinder its cost-effective implementation for high volume production of metal products. Importantly, the length of a workpiece for processing by ECAE is limited by the stroke distance of the ECAE die press used for processing, and the length/diameter ratio is limited because a large length/diameter ratio makes the extrusion unstable. Furthermore, the ECAE method is currently a discontinuous, slow, and labor intensive, which makes the resulting UFG products expensive.
Clearly, a method for refining the grain size of a workpiece without significantly changing its dimensions is highly desirable. Therefore, an object of the present invention is a method for refining the grain size of a workpiece without significantly changing its dimensions.
Another object of the present invention is a method of improving the hardness and strength properties of a workpiece without significantly changing its dimensions.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.